Ink Cups & Doctor Rings

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Inkcup and Doctor Ring
Types and Applications

For a number of years the market for pad printing supplies has experienced widespread use of sealed inkcup systems over open inkwell doctor blade systems. Addressing the tremendous amount of interest regarding this kind of pad printing supplies, the article below discusses inkcup types available in the marketplace, the advantages of each system, the relative benefits in the types of doctor ring, and doctor ring applications.

Inkcup Types

A sealed inkcup system is essentially and inverted inkcup that is filled with ink and uses the sharp rim of the cup as a printing plate wiping system. As this system floods and doctors (wipes) a printing plate in the same motion, solvent evaporation and ink viscosity changes are much more limited than with doctor blade systems, making the sealed inkcup printing process significantly more predictable and manageable.

Nearly all sealed inkcup types can be broken into two categories, magnetic or non-magnetic. Both inkcup types, when integrated into the pad printing machine yield excellent results for clean, properly doctored printing plates.

A magnetic inkcup is basically “self-doctoring unit”. With its extremely powerful magnets, it has all the necessary down-force to clean the plate requiring the pad printing machine to simply pull it back and forth. The magnetic inkcup generally has found its strength in multi-color applications because the pad printing equipment can generally be of simple construction and the multi-color pad printer more cost effective.Non-magnetic inkcups are designed to have the pressure applied by the pad printing machine to an outer flange or centering hole in the cup. The pressure is applied by a spring-loaded system geared for one inkcup assembly. The spring-loaded system is compact and very effective, but is not as simple to deploy on a multi-color pad printing machines. The non-magnetic inkcups seem to have their niche in compact or high speed machine applications. In these systems, even at high speeds, the spring loaded hold-down system keeps the inkcup firmly in place while the plate moves in and out.

Doctor Rings

There has been much debate as to which doctor ring material is best for doctoring printing plates. The market for pad printing supplies originally started with carbide steel doctor rings and many were successfully implemented, however, since the ceramic ring was brought to the US market in 1993 it has been extremely successful as well. We would guess that now the pad printing marketplace is evenly divided between the carbide ring and the new generation ceramic doctor ring. Today, both carbide rings and ceramic rings work extremely well. It is our opinion however, that the ceramic doctor ring offers some advantages.

One of the primary advantages of the ceramic ring has a “self-lubricating” qualities that make I work equally well on thin steel and thick steel and the softer polymer printing plates. The carbide doctor rings are generally fairly sharp and abrasive and will wear a polymer plate material quickly. Another advantage of the ceramic ring is that it is generally thicker, more robust and resistant to damage by during handling. Most ceramic rings taper from the inside and outside diameter to a “point” roughly .006” leaving plenty of material near the tip. Carbide rings, however, taper from the outside diameter only to a “point” with a cross section of .003” leaving only a minimum amount of material near the tip resulting in more damage during operator handling. The combination of the ring profile and the brittle nature of carbide make damage to rings fairly frequent during ordinary use.

In general, the quality of pad printing supplies has become very high, so any type of sealed inkcup or ring material allows making excellent imprints.